The present application relates to a process for the treatment of waste containing asbestos, originating from a site generating such waste, including a reaction by digestion of the waste with a basic solution until a reaction product substantially without asbestos fiber is obtained.
The harmful effects of asbestos in the human respiratory tract are well known. Most of the countries in the world make provisions for stripping down asbestos, in particular in building construction. At present, when stripping down is carried out, the asbestos is stored in double bags and is subsequently transported to treatment centers.
The largest treatment centers carry out one of two alternate solutions, either (a) coating with hydraulic binders and dumping of the product obtained, or (b) incineration by vitrification at very high temperature.
In both cases, there are big disadvantages. For example, there are the hazards linked with the transport and storage, during which the smallest incident can result in pollution of the environment. Furthermore, alternate solution (a) noted above merely postpones the problem, since it does not destroy the asbestos fibers; alternate solution (b) noted above is very costly, and both solutions (a) and (b) noted above do not provide for any reclaiming of the products resulting from the treatment.
Treatments with acids are also known, which have the disadvantage of polluting the environment with other hazardous wastes.
Lastly, there are known treatments of waste containing asbestos in alkaline medium.
WO-A-03/18867 describes a process in which the waste is ground very finely in the presence of at least one substance which releases OHxe2x88x92 ions in water, so as to form an aqueous suspension which is optionally subsequently transferred to an autoclave and treated at elevated temperature and pressure. The process described in WO-A-03/18867 has the disadvantage of a preliminary stage of very fine grinding, which requires a very powerful, and hence stationary, plant. The process therefore does not solve the problems which are inherent in the transport and storage of waste, and appears to be very costly. Furthermore, nothing is provided with regard to the fumes and liquid effluents which are released during the process and which, in their turn, run the risk of polluting the environment.
WO-A-94/08661 describes a treatment process as indicated at the beginning. The only aim of this process is to produce waste that can be tipped on a dump without the hazards inherent to asbestos. The only plant described for carrying out this treatment is a stationary treatment center, of large size, which therefore does not provide any solution to the problems of transport and storage of waste containing asbestos. Finally, during the treatment the waste is subjected to a compacting operation which is costly in energy, before being introduced into the treatment chamber.
The objective of the present invention is to provide a process and a plant which avoid the above-mentioned disadvantages and which permit treatment of the waste on the site which generates such waste. A site which generates waste should be understood to mean not only a building in which all the components containing asbestos are stripped down, but also, for example, a refuse depot where bags containing asbestos have been previously accumulated. It is therefore desirable that the plant should be of small size and transportable.
A further objective of the invention is to avoid any risk of pollution between the waste generating site and the reactor which will treat the waste, that is to say, to avoid the risks of bagging of the waste and of the transport of these bags towards a distant treatment center. The bagging of structural debris is an awkward operation and very frequently results in the bags being perforated, which subsequently allows asbestos to be spread around in the atmosphere outside the site.
Another objective of the invention is to prevent as much as possible any escape of product involved in the treatment, insofar as it still contains asbestos fibers. The products resulting from the treatment will be advantageously either recycled or are suitable for being reclaimed.
Finally, another objective of the invention is to avoid any grinding, crushing or compacting of the waste prior to the entry into the reactor and hence an introduction into the latter, without any sorting, of substrates laden with asbestos.
These problems are solved, in accordance with the invention, by a process as generally described as including various combinations of the steps of, and the apparatus for:
a transfer of the waste containing asbestos from the generating site into the interior of a treatment reactor, without the waste being in contact with an external medium;
feeding the reactor with basic digestion solution until a reaction product is obtained, substantially without fibers;
separation of the reaction product into a solid phase and a liquid phase;
recycling of the liquid phase resulting from the aforementioned separation, and optionally of a recycling gaseous medium drawn off from the reactor after the digestion, in order to form the basic digestion solution;
recovery of the solid phase resulting from the separation, with a view to an optional reclaiming; and
discharge out of the reactor of a small quantity of irrecoverable waste containing no asbestos fibers.
Furthermore the above-mentioned stages are carried out in a xe2x80x9cclosedxe2x80x9d circuit without any possible outward release of asbestos fibers, with any product resulting from this circuit being substantially devoid of any asbestos fiber.
In accordance with one advantageous embodiment of the invention, the transfer of the waste containing asbestos takes place in a closed, removable receptacle and the process additionally includes, during or after the transfer, an external washing of the receptacle with a washing liquid. This makes it possible to eliminate the asbestos dust which may have been deposited on the receptacle during its presence at the waste generating site, and hence not to contaminate the location, near the site, where the basic digestion is to take place. The water from washing the receptacle is preferably recycled into the formation of a basic digestion solution, and this prevents any pollution of the environment when the closed receptacle leaves the place which generates waste.
In accordance with a particularly advantageous aspect of the invention, the removable receptacle used for the above-mentioned transfer is also the reactor in which the digestion reaction takes place. In this case, there is therefore no risk of pollution by the asbestos during the transfer from one receptacle into another; the unsorted waste is introduced directly into the reactor at the waste generating site itself.
According to one embodiment of the invention the digestion reaction is performed with the basic digestion solution at a temperature of the order of 175xc2x0 to 220xc2x0 C. and at a pressure of approximately 3 to 10 kg/cm2. More preferably, the digestion reaction is performed with the basic digestion solution at a temperature in the order of 190xc2x0 to 220xc2x0 C. and at a pressure of approximately 3 to 5 kg/cm2.
According to an alternate embodiment of the invention the digestion includes:
a first digestion in the reactor with the basic digestion solution at a temperature which makes it possible to obtain a product of disassociation of the asbestos fibers from the other waste, while maintaining a low pressure in the reactor;
a separation of the above-mentioned product of disassociation into a solid phase which is concentrated in volume in relation to the waste containing asbestos transferred into the reactor, and a liquid phase;
recycling of the liquid phase for the formation of the basic digestion solution;
introduction of the solid phase concentrated in volume, originating from the product of disassociation, into an additional reactor;
a second digestion, in the additional reactor, of this solid phase concentrated in volume with the basic digestion solution, at a temperature and a pressure which are sufficiently elevated to obtain the said reaction product without fibers; and
cooling of the said reaction product before its above-mentioned separation.
This method of treatment makes it possible, in a first stage, to reduce the bulk of the waste containing asbestos. For example, asbestos waste from flocking is generally, when dismantled, in the form of wadding whose specific weight is of the order of 150 to 300 g per liter. During the first digestion, performed at a relatively moderate temperature, it is possible to employ reactors such as autoclaves, which are much less heavy and above all less costly because of the low pressure used. In the second digestion stage, the solid phase which is treated has a volume which has been reduced by 70% in relation to the volume of the waste used in the first digestion stage. The second reactor employed can then subject this solid phase to an elevated temperature which is sufficient to make the asbestos fibers disappear without necessarily having to reach an exaggeratedly elevated pressure in this second reactor. The yield can thus be increased, while the cost of the removable reactors is drastically reduced.
Other embodiments of the process in accordance with the invention are described herein.
In accordance with the invention the problems presented have also been solved with an apparatus for implementing the process in accordance with the invention and the apparatus is fully described herein.